1. Field of the Invention
The present invention relates to an exposure apparatus which exposes a substrate to light via a projection optical system, and a method of manufacturing a device using the exposure apparatus.
2. Description of the Related Art
Exposure apparatuses for transferring the patterns of originals (reticles) onto substrates (wafers) are classified into a single-stage type and a twin-stage type that differ in the number of stages which hold substrates (Japanese Patent Laid-Open No. 2000-323404).
A single-stage type exposure apparatus can perform substrate surface shape (surface position) measurement (to be referred to as focus measurement hereinafter) for focus control, immediately before exposure in each shot or in parallel with exposure in each shot. FIG. 2A schematically shows exposure and focus measurement in a single-stage type exposure apparatus. Referring to FIG. 2A, the layout (shot layout) of shot regions 202A is defined for a substrate 201A. A single-stage type scanning exposure apparatus (scanner), for example, performs focus measurement in parallel with exposure in a first shot region 203, and performs focus measurement again in parallel with exposure in a second shot region 204. An exposure apparatus of the stepper type performs focus measurement immediately before exposure in each shot.
A twin-stage type exposure apparatus measures, e.g., undulations and strain of a substrate in a measurement station, and uses the obtained measurement result in exposing the substrate in an exposure station. Hence, the twin-stage type exposure apparatus can measure the above-mentioned characteristics of the substrate surface free from any limitations imposed on the substrate by the shot layout. The twin-stage type exposure apparatus typically successively measures the surface shapes of a substrate in a plurality of shot regions while scanning the substrate. This method is commonly called column measurement. FIG. 2B schematically shows exposure and focus measurement in a twin-stage type exposure apparatus. Referring to FIG. 2B, the layout (shot layout) of shot regions 202B is defined for a substrate 201B. The twin-stage type exposure apparatus performs column measurement 205 of the first column, and then performs column measurement 206 of the second column.
To determine the surface shape of a substrate having a pattern, one general method is to calculate the characteristics of the substrate surface by taking account of (by eliminating) pattern undulations. Referring to FIG. 5A, the surface shapes of a substrate having pattern undulations 501A are measured in measurement regions 502A, 503A, and 504A. Focus measurement operations 505 and 506 at a plurality of positions in the measurement regions 502A, 503A, and 504A will be considered. In this case, if the measurement values are used directly, the substrate is exposed upon being erroneously regarded as having surfaces 507A, 508A, and 509A.
To prevent this, pattern undulations are measured in advance and used as an offset (pattern offset). With this operation, the surface of the substrate having the pattern undulations 501A (that are measured in measurement regions 502B, 503B, and 504B) can be positioned with respect to a reference plane 510 (the image plane of a projection optical system), as shown in FIG. 5B.
Also, the mainstream stage control and focus measurement employ separate processors because of factors associated with a processing load. Therefore, it is a common practice to perform focus measurement while driving the stage by detecting the stage position once, and thereafter performing sampling and measurement at a predetermined time interval by the focus processor. For example, as shown in FIG. 5C, focus measurement in scanning exposure of shot regions 511, 512, and 513 in the directions indicated by arrows is started from, e.g., measurement regions 502C of 502C, 503C and 504C, 504D of 502D, 503D and 504D, and 502E of 502E, 503E and 504E, respectively, to measure the surface positions at positions having the same underlying portions among the shot regions 511, 512, and 513.
In column measurement for successively measuring a plurality of shot regions, the shot layout pitch cannot always be divided by a sampling pitch, which is determined by the sampling timing, without a remainder. In this respect, the focus measurement position may differ for each shot region. For example, as shown in FIG. 5D, two layouts 517A and 517B having shot regions with different lengths in the scanning direction will be considered. In a layout 514A, the layout pitch of shot regions is an integer multiple of the sampling pitch. In a layout 514B, the layout pitch of shot regions is not an integer multiple of the sampling pitch. The layout 517A allows surface position measurement at identical measurement points throughout all shot regions. In contrast to this, the layout 517B does not allow measurement points in each shot region to be identical throughout all shot regions. It is therefore hard to calculate and use a pattern offset in a case like the layout 517B.